A Walking Tour Through the Brain
By Michael Murphy on July 12th, 2008(viewed 4003 times).

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Part 1: A Walking Tour Through the Brain

The brain is a remarkable organ. Seemingly without any effort, it allows us to carry out every element of our daily lives. It manages many of the body functions that happen without our knowledge or direction, such as breathing, blood circulation, and digestion. It also directs all the functions we carry out consciously. We can speak, move, see, remember, feel emotions, and make decisions because of the complicated mix of chemical and electrical processes that take place in our brains.

The Brain's Vital Statistics
  • Adult weight: about 3 pounds
  • Adult size: a medium cauliflower
  • Number of neurons: 100,000,000,000 (100 billion)
  • Number of synapses (the gap between neurons): 100,000,000,000,000 (100 trillion)

Our brains are made of nerve cells and lots of other cell types. Nerve cells are also called neurons. The neurons of all animals function in basically the same way, even though animals can be very different from each other. What sets people apart from other animals is the huge number of nerve cells we have in the cerebral cortex, regions of which are proportionally much larger in humans than in any other animals. These regions are the parts of the brain where cognitive functions, like thinking, learning, speaking, remembering, and making decisions, take place. The many interconnections among the nerve cells in these regions also make us different from other animals.

To understand Alzheimer's disease, it's important to know a bit about the brain. Part 1 of Unraveling the Mystery first gives an inside view of the normal brain, how it works, and what happens during aging. Then, it shows what happens to the brain in Alzheimer's and how the disease slowly destroys a person's mental and physical capacities.

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Inside the Human Brain

The Three Main Players Diagram of the brainThe cerebral hemispheres accounts for 85 percent of the brain's weight. The billions of neurons in the two hemispheres are connected by a thick bundle of nerves called the corpus callosum. Scientists now think that the two hemispheres differ not so much in what they focus on (the "logical versus artistic" notion), but how they process information. The left hemisphere appears to focus on the details (such as recognizing a particular face in a crowd). The right hemisphere focuses on the broad background (such as understanding the relative position of objects in a space). The cerebral hemispheres have an outer layer called the cerebral cortex. This is where the brain processes sensory information received from the outside world, controls voluntary movement, and regulates conscious thought and mental activity.

The cerebellum takes up a little more than 10 percent of the brain. It's in charge of balance and coordination. The cerebellum also has two hemispheres. They are always receiving information from the eyes, ears, and muscles and joints about the body's movements and position. Once the cerebellum processes the information, it works through the rest of the brain and spinal cord to send out instructions to the body. The cerebellum's work allows us to walk smoothly, maintain our balance, and turn around without even thinking about it.

The brain stem sits at the base of the brain. It connects the spinal cord with the rest of the brain. Even though it's the smallest of the three main players, its functions are crucial to survival. The brain stem controls the functions that happen automatically to keep us alive - our heart rate, blood pressure, and breathing. It also relays information between the brain and the spinal cord, which then sends out messages to the muscles, skin, and other organs. Sleep and dreaming are also controlled by the brain stem.

Other Crucial Parts Several other essential parts of the brain lie deep inside the cerebral hemispheres:

  • The limbic system links the brain stem with the higher reasoning elements of the cerebral cortex. It controls emotions and instinctive behavior. This is also where the sense of smell is located.


  • The hippocampus is important for learning and short-term memory. This part of the brain is considered to be the site where short-term memories are converted into long-term memories for storage in other brain areas.


  • The thalamus receives sensory and limbic information, processes it, and then sends it to the cerebral cortex.


  • The hypothalamus is a structure under the thalamus that monitors activities like body temperature and food intake. It issues instructions to correct any imbalances. The hypothalamus also controls the body's internal clock.

The Brain in Action New imaging techniques allow scientists to monitor brain function in living people. This is opening up worlds of knowledge about normal brain function and how it changes with age or disease.

One of these techniques is called positron emission tomography, or PET scanning. PET scans measure blood flow and glucose metabolism throughout the brain. (For more on metabolism see the section Neurons and Their Jobs) When nerve cells in a region of the brain become active, blood flow and metabolism in that region increase. These increases are usually shown as red and yellow colors on a PET scan. Shades of blue and black indicate decreased or no activity within a brain region. In essence, a PET scan produces a "map" of the active brain.

Brain scans

The Aging Brain As a person gets older, changes occur in all parts of the body, including the brain:

  • Some neurons shrink, especially large ones in areas important to learning, memory, planning, and other complex mental activities.
  • Tangles and plaques develop in neurons and surrounding areas, though in much smaller amounts than in AD (see the section Plaques and Tangles for more information).
  • Damage by free radicals increases (free radicals are a kind of molecule that reacts easily with other molecules; see the section Oxidative Damage from Free Radicals for more on these molecules).

PET scans

What is the impact of these changes? Healthy older people may notice a modest decline in their ability to learn new things and retrieve information, such as remembering names. They may perform worse on complex tasks of attention, learning, and memory. However, if given enough time to perform the task, the scores of healthy people in their 70s and 80s are often the same as those of young adults. As they age, adults often improve their vocabulary and other forms of verbal knowledge.


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Neurons and Their Jobs

Neurons and Their Jobs The human brain is made up of billions of neurons. Each has a cell body, an axon, and many dendrites. The cell body contains a nucleus, which controls all of the cell's activities, and several other structures that perform specific functions. The axon, which is much, much narrower than the width of a human hair, extends out from the cell body and transmits messages to other neurons. Sometimes, the messages have to travel over very long distances (even up to 5 feet!). Dendrites also branch out from the cell body. They receive messages from the axons of other nerve cells. Each nerve cell is connected to thousands of other nerve cells through its axon and dendrites. Neurons are surrounded by glial cells, which support, protect, and nourish them.

Groups of neurons in the brain have special jobs. For example, some are involved with thinking, learning, and memory. Others are responsible for receiving sensory information. Still others communicate with muscles, stimulating them into action.

Several processes all have to work smoothly together for neurons to survive and stay healthy. These processes are communication, metabolism, and repair.

  Diagram of neurons
  Click here for larger view
Communication: Sending Millions of Messages a Second Imagine the telecommunication cables that run under our streets. All day and night, millions of telephone calls are flashing down fiber optic cables at incredible speeds, letting people strike deals, give instructions, share a laugh, or learn some news. Multiply that many-fold and that's the brain. Neurons are the great communicators, always in touch with their neighbors.

As a neuron receives messages from surrounding cells, an electrical charge, or nerve impulse, builds up. This charge travels down the axon until it reaches the end. Here, it triggers the release of chemical messengers called neurotransmitters, which move from the axon across a tiny gap to the dendrites or cell bodies of other neurons. The typical neuron has up to 15,000 of these tiny gaps, or synapses. After they move across the synapse, neurotransmitters bind to specific receptor sites on the receiving end of dendrites of the nearby neurons. They can also bind directly to cell bodies.

Once the receptors are activated, they open channels through the cell membrane into the receiving nerve cell's interior or start other processes that determine what the receiving nerve cell will do. Some neurotransmitters inhibit nerve cell function (that is, they make it less likely that the nerve cell will send an electrical signal down its axon). Other neurotransmitters stimulate nerve cells; they prime the receiving cell to become active or send an electrical signal down the axon to more neurons in the pathway.

During any one moment, millions of these signals are speeding through pathways in the brain, allowing it to receive and process information, make adjustments, and send out instructions to various parts of the body. If neurons are disconnected, they become sick and may die.

  Click here for larger view
Metabolism: Turning Chemicals and Nutrients Into Energy to Keep Neurons Working Metabolism is the process by which cells and molecules break down chemicals and nutrients to generate energy and form building blocks that make new cellular molecules like proteins. Efficient metabolism needs enough blood circulating to supply the cells with oxygen and glucose, a type of sugar. Glucose is the only source of energy usually available to the brain. Without oxygen or glucose, neurons will die.

This figure shows young and aged rat neurons at rest and with increasing duration of stimulation. When neurons are stimulated, metabolism increases. The stimulated neurons of young rats maintain calcium within normal levels. Older rats are unable to do this. High levels of calcium in old neurons may make them susceptible to dysfunction and death. The color scale is an index of cellular calcium with red indicating the highest levels.

  Image of neuron repair
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Repair: Keeping Long-lived Neurons in Good Working Order Unlike most cells, which have a fairly short lifespan, nerve cells, which are generated in the fetus or a short time after birth, live a long time. Brain neurons can live for up to 100 years or longer. In an adult, when neurons die because of disease or injury, theyare not usually replaced. Recent research, however, shows that in a few brain regions, new neurons can be born, even in the old brain.

To prevent their own death, living neurons must constantly maintain and remodel themselves. If cell cleanup and repair slows down or stops for any reason, the nerve cell cannot function well. Eventually, it dies.

This figure shows the effects of exercise on levels of brain-derived neurotrophic factor (BDNF) in the hippocampus of rats. Growth factors like BDNF help many neurons survive. Levels of the message that makes BDNF are much higher in exercising rats (a) than in sedentary animals (b). Exercise may promote healthy neurons in rats by causing their neurons to make more protective BDNF. Red and yellow denote the highest levels of BDNF, while green and blue denote the lowest.

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Plaques and Tangles: The Hallmarks of AD

Plaques and Tangles: The Hallmarks of AD Alzheimer's disease disrupts each of the three processes that keep neurons healthy: communication, metabolism, and repair. This disruption causes certain nerve cells in the brain to stop working, lose connections with other nerve cells, and finally, die. The destruction and death of nerve cells causes the memory failure, personality changes, problems in carrying out daily activities, and other features of the disease.

The brains of AD patients have an abundance of two abnormal structures - beta amyloid plaques and neurofibrillary tangles. This is especially true in certain regions of the brain that are important in memory. Plaques are dense, mostly insoluble (cannot be dissolved) deposits of protein and cellular material outside and around the neurons. Tangles are insoluble twisted fibers that build up inside the nerve cell. Though many older people develop some plaques and tangles, the brains of AD patients have them to a much greater extent. Scientists have known about plaques and tangles for many years, but recent research has shown much about what they are made of, how they form, and their possible roles in AD.

Amyloid PlaquesAmyloid Plaques Plaques are made of beta-amyloid, a protein fragment snipped from a larger protein called amyloid precursor protein (APP). These fragments clump together and are mixed with other molecules, neurons, and non-nerve cells. In AD, plaques develop in the hippocampus, a structure deep in the brain that helps to encode memories, and in other areas of the cerebral cortex that are used in thinking and making decisions. We still don't know whether beta-amyloid plaques themselves cause AD or whether they are a by-product of the AD process. We do know that changes in APP structure can cause a rare, inherited form of AD (see the section Genes and Early-Onset Alzheimer's Disease for more on inherited AD).

From APP to Beta-amyloid APP is a protein that appears to be important in helping neurons grow and survive. APP may help damaged neurons repair themselves and may help parts of neurons grow after brain injury. In AD, something causes APP to be snipped into fragments, one of which is called beta-amyloid; the beta-amyloid fragments eventually clump together into plaques.

APP is associated with the cell membrane, the thin barrier that encloses the cell. After it is made, APP sticks through the neuron's membrane, partly inside and partly outside the cell.

Enzymes (substances that cause or speed up a chemical reaction) act on the APP and cut it into fragments of protein, one of which is called beta-amyloid.

The beta-amyloid fragments begin coming together into clumps outside the cell, then join other molecules and non-nerve cells to form insoluble plaques.


Click here for larger view   Click here for larger view
APP is associated with the cell membrane, the thin barrier that encloses the cell. After it is made, APP sticks through the neuron's membrane, partly inside and partly outside the cell.   Enzymes (substances that cause or speed up a chemical reaction) act on the APP and cut it into fragments of protein, one of which is called beta-amyloid.

beta amyloid fragments

Click here for larger view    
The beta amyloid fragments begin coming together into clumps outside the cell, then join other molecules and non-nerve cells to form insoluable plaques.    

Neurofibrillary Tangles Healthy neurons have an internal support structure partly made up of structures called microtubules. These microtubules act like tracks, guiding nutrients and molecules from the body of the cell down to the ends of the axon and back. A special kind of protein, tau, makes the microtubules stable. In AD, tau is changed chemically. It begins to pair with other threads of tau and they become tangled up together. When this happens, the microtubules disintegrate, collapsing the neuron's transport system. This may result first in malfunctions in communication between neurons and later in the death of the cells.

Neurofibrillary Tangles Click here for larger view


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The Changing Brain in Alzheimer's Disease  

The Changing Brain in Alzheimer's Disease No one knows exactly what causes the Alzheimer's disease process to begin or why some of the normal changes associated with aging become so much more extreme and destructive in patients with the disease. We do know a lot, however, about what happens in the brain once AD takes hold and about the physical and mental changes that occur over time. The time from diagnosis to death varies - as little as 3 years if the patient is over 80 when diagnosed, as long as 10 or more years if the patient is younger. Although the course of AD is not the same in every patient, symptoms seem to develop over the same general stages.

PET Scan of Normal Brain   PET Scan of Alzheimer's Disease Brain
PET Scan of Normal Brain   PET Scan of Alzheimer's Disease Brain
  Preclinical AD
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Preclinical AD AD begins in the entorhinal cortex, which is near the hippocampus and has direct connections to it. It then proceeds to the hippocampus, the structure that is essential to the formation of short-term and long-term memories. Affected regions begin to atrophy (shrink). These brain changes probably start 10 to 20 years before any visible signs and symptoms appear. Memory loss, the first visible sign, is the main feature of mild cognitive impairment (MCI) (see the section Criteria for "Probable" Alzheimer's Disease for more on MCI). Many scientists think MCI is often an initial, transitional phase between normal brain aging and AD.

  Mild AD
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Mild AD As the disease begins to affect the cerebral cortex, memory loss continues and changes in other cognitive abilities emerge. The clinical diagnosis of AD is usually made during this stage. Signs of mild AD can include:
  • Memory loss
  • Confusion about the location of familiar places (getting lost begins to occur)
  • Taking longer to accomplish normal daily tasks
  • Trouble handling money and paying bills
  • Poor judgment leading to bad decisions
  • Loss of spontaneity and sense of initiative
  • Mood and personality changes, increased anxiety

The growing number of plaques and tangles first damage areas of brain that control memory, language, and reasoning. It is not until later in the disease that physical abilities decline. This leads to a situation in mild AD in which a person seems to be healthy, but is actually having more and more trouble making sense of the world around him or her. The realization that something is wrong often comes gradually because the early signs can be confused with changes that can happen normally with aging. Accepting these signs and deciding to go for diagnostic tests can be a big hurdle for patients and families to cross.


  • Increasing memory loss and confusion
  • Shortened attention span
  • Problems recognizing friends and family members
  • Difficulty with language; problems with reading, writing, working with numbers
  • Difficulty organizing thoughts and thinking logically
  • Inability to learn new things or to cope with new or unexpected situations
  • Restlessness, agitation, anxiety, tearfulness, wandering - especially in the late afternoon or at night
  • Repetitive statements or movement, occasional muscle twitches
  • Hallucinations, delusions, suspiciousness or paranoia, irritabilityLoss of impulse control (shown through sloppy table manners, undressing at
  • inappropriate times or places, or vulgar language)
  • Perceptual-motor problems (such as trouble getting out of a chair or setting the table)

Behavior is the result of complex brain processes, all of which take place in a fraction of a second in the healthy brain. In AD, many of these processes are disturbed, and this is the basis for many distressing or inappropriate behaviors. For example, a person may angrily refuse to take a bath or get dressed because he does not understand what his caregiver has asked him to do. If he does understand, he may not remember how to do it. The anger is a mask for his confusion and anxiety. Or, a person with AD may constantly follow her husband or caregiver and fret when the person is out of sight. To a person who cannot remember the past or anticipate the future, the world around her can be strange and frightening. Sticking close to a trusted and familiar caregiver may be the only thing that makes sense and provides security. Taking off clothes may seem reasonable to a person with AD who feels hot and doesn't understand or remember that undressing in public is not acceptable.

  Severe AD
  Click here for larger view
Severe AD In the last stage of AD, plaques and tangles are widespread throughout the brain, and areas of the brain have atrophied further. Patients cannot recognize family and loved ones or communicate in any way. They are completely dependent on others for care. All sense of self seems to vanish. Other symptoms can include:
  • Weight loss
  • Seizures, skin infections, difficulty swallowing
  • Groaning, moaning, or grunting
  • Increased sleeping
  • Lack of bladder and bowel control

At the end, patients may be in bed much or all of the time. Most people with AD die from other illnesses, frequently aspiration pneumonia. This type of pneumonia happens when a person is not able to swallow properly and breathes food or liquids into the lungs.

Preclinical AD   Mild to Moderate AD   Severe AD
Preclinical AD Click here for larger view   Mild to Moderate AD Click here for larger view   Severe AD Click here for larger view
Moderate AD By this stage, AD damage has spread further to the areas of the cerebral cortex that control language, reasoning, sensory processing, and conscious thought. Affected regions continue to atrophy and signs and symptoms of the disease become more pronounced and widespread. Behavior problems, such as wandering and agitation, can occur. More intensive supervision and care become necessary, and this can be difficult for many spouses and families. The symptoms of this stage can include:


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Finding New Answers and Asking Better Questions

In the past 25 years, scientists have studied Alzheimer's disease from many angles. They've looked at populations to see how many cases of AD occur and whether there might be links between the disease and lifestyles or genetic backgrounds. They've conducted clinical studies with healthy older people and those at various stages of AD. They've examined individual nerve cells to see how beta-amyloid and other molecules affect the ability of cells to function normally.

These studies have led to better diagnostic tests, new ways to manage behavioral aspects of AD, and a growing number of possible drug treatments. Findings from current research are pointing scientists in promising directions for the future. They are also helping researchers ask better questions about the issues that are still unclear.

Part 2 of Unraveling the Mystery describes what we're learning from our search for:

  • The causes of AD
  • New techniques to help in diagnosis
  • New treatments
  • Ways to improve support for families and other caregivers

Results from this research will bring us closer to the day when we will be able to prevent or even cure the devastating disease that robs our older relatives and friends of their most precious possession - their minds.

Then and Now: the Fast Pace of Development in AD Research

What We Didn't Know Then 15 years ago

  • We didn't know any of the genes that could cause AD.
  • We had no idea of the biological pathways that were involved in the development of damage to the brain in AD.

10 years ago

  • We couldn't model the disease in animals

5 years ago

  • NIH did not fund any prevention clincial trials.
  • We had no way to identify people at high risk of developing AD.

1 year ago

  • We didn't understand anything about how plaques and tangles relate to each other.

What We Know Now (2002)

  • We know the 3 major genes for early-onset AD adn 1 of the major risk factor genes for late-onset AD.
  • We know a lot about the pathways that lead to the development of beta-amyloid plaques in the brain — one of the main features of AD.
  • Scientists have developed special kinds of mice that produce beta-amyloid plaques.
  • NIH is funding clinical trials that are looking at possible ways to prevent AD.
  • We can identify individuals at high risk through imaging, neuropsychological tests, and structured interviews.
  • By developing another kind of mice that have both plaques and tangles, we now know that plaques can influence the development of tangles.

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A. The Search for Causes

Scientists working in a laboratoryOne of the most important parts of unraveling the AD mystery is finding out what causes the disease. What makes the disease process begin in the first place? What makes it worse over time? Why does the number of people with the disease increase with age? Why does one person develop it and another remain healthy?

Some diseases, like measles or pneumonia, have clear-cut causes. They can be prevented with vaccines or cured with antibiotics. Others, such as diabetes or arthritis, develop when genetic, lifestyle, and environmental factors work together to cause a disease process to start. The importance of each one of these factors may be different for each individual.

AD fits into this second group of diseases. We don't yet fully understand what causes AD, but we know it develops because of a complex series of events that take place in the brain over a long period of time. Many studies are exploring the factors involved in the cause and development of AD.

Genetic Factors at Work in AD In the last few years, painstaking detective work by scientists has paid off in discoveries of genetic links to the two main types of AD. One type is the more rare, early-onset Alzheimer's disease. It usually affects people aged 30 to 60. Some cases of early-onset disease are inherited and are called familial AD (FAD). The other is late-onset Alzheimer's disease. It is the most common form and occurs in those 65 and older


DNA, Chromosomes, and Genes: The Body's Amazing Control Center
Diagram of DNA
Click here for larger view

The nucleus of almost every human cell contains a vast chemical information database. This database carries all the instructions the cell needs to do its job. This database is DNA. DNA exists as two long, intertwined, thread-like strands packaged in units called chromosomes. Each cell has 46 chromosomes in 23 pairs. Chromosomes are made up of four chemicals, or bases, arranged in various sequence patterns. People inherit material in each chromosome from each parent.

Each chromosome has many thousands of segments, called genes. The sequence of bases in a gene tells the cell how to make specific proteins. Proteins determine the physical characteristics of living organisms. They also direct almost every aspect of the organism's construction, operation, and repair. Even slight alterations in a gene can produce an abnormal protein, which, in turn, can lead to cell malfunction, and eventually, to disease. Any rare change in a gene's DNA that causes a disease is called a mutation. Other more common (or frequent) changes in a gene's DNA don't automatically cause disease, but they can increase the chances that a person will develop a particular disease. When this happens, the changed gene is called a genetic risk factor.


Genes and Early-onset Alzheimer's Disease

Scientist working in a laboratoryEven though early-onset AD is very rare and mutations in these three genes do not play a role in the more common late-onset AD, these findings were crucial because they showed that genetics was indeed a factor in AD, and they helped to identify some key players in the AD disease process. Importantly, they showed that mutations in APP can cause AD, highlighting the key role of beta-amyloid in the disease. Many scientists believe that mutations in each of these genes cause an increased amount of the damaging beta-amyloid to be made in the brain.

The findings also laid the foundation for many other studies that have pushed back the boundaries of our knowledge and created new possibilities for future treatment. For example, in the last several years, a series of highly sophisticated experiments have shown that presenilin may actually be one of the enzymes (substances that cause or speed up a chemical reaction) that clips APP to form beta-amyloid (the protein fragment that is the main component of AD plaques). This discovery has helped clarify how presenilins might be involved in the early stages of AD. It has also given scientists crucial new targets for drug therapy and has spurred many new studies in the test tube, in animals, and even in people.

A Different Genetic Story in Late-onset Alzheimer's Disease While some scientists were focused on the role of chromosomes 21, 14, and 1 in early-onset AD, others were looking elsewhere to see if they could find genetic clues for the late-onset form. By 1992, these investigators had narrowed their search to a region of chromosome 19. At the same time, other colleagues were looking for proteins that bind to beta-amyloid. They were hoping to clarify some of the steps in the very early stages of the disease process. They found that one form of a protein called apolipoprotein E (ApoE) did bind quickly and tightly to beta-amyloid. They also found that the gene that produces ApoE was located in the same region of chromosome 19 pinpointed by the geneticists. This finding led them to suggest that one form of this gene was a risk factor for late-onset Alzheimer's disease.

Other studies since then have shown that the gene that produces ApoE comes in several forms, or alleles - e2, e3, and e4. The APOE e2 allele is relatively rare and may provide some protection against the disease. If AD does occur in a person with this allele, it develops later in life. APOE e3 is the most common allele. Researchers think it plays a neutral role in AD. APOE e4 occurs in about 40 percent of all AD patients who develop the disease in later life. It is not limited to people whose families have a history of AD, though. AD patients with no known family history of the disease are also more likely to have an APOE e4 allele than persons who do not have AD. Dozens of studies have confirmed that the APOE e4 allele increases the risk of developing AD. These studies have also helped to explain some of the variation in the age at which AD develops. However, inheriting an APOE e4 allele doesn't mean that a person will definitely develop AD. Some people with one or two APOE e4 alleles never get the disease and others who do develop AD do not have any APOE e4 alleles.

Although we still don't exactly know how APOE e4 increases AD risk, one theory is that when its protein product binds quickly and tightly to beta-amyloid, the normally soluble amyloid becomes insoluble. This may mean that it is more likely to be deposited in plaques.

While scientists are working to understand more fully the APOE gene and its role in AD, they have also identified regions on other chromosomes that might contain genetic risk factors. For example, in 2000, three teams of scientists, using three different strategies, published studies showing that chromosome 10 has a region that may contain several genes that might increase a person's risk of AD. Identifying these genes is one important step in the research process that will lead to new understanding about the ways in which changes in protein structures cause the disease process to begin and the sequence of events that occurs as the disease develops. Once they understand these processes, scientists can search for new ways to diagnose, treat, or even prevent AD.

Other Factors at Work in AD Even if genetics explains some of what might cause AD, it doesn't explain everything. So, researchers have looked at other possibilities that may reveal how the Alzheimer's disease process starts and develops.

Beta-amyloid We still don't know whether beta-amyloid plaques cause AD or whether they are a by-product of the disease process. We do know, however, that forming beta-amyloid from APP is a key process in AD. That's why finding out more about beta-amyloid is an important avenue of ongoing AD research. Investigators are studying:

  • The nature of beta-amyloid
  • Ways in which it is toxic to neurons
  • Ways in which plaques form and are deposited
  • Ways in which beta-amyloid and plaques might be reduced in the brain

TauTau In the last few years, scientists have been giving an increasing amount of attention to tau, the other hallmark of Alzheimer's disease. This protein is commonly found in nerve cells throughout the brain. In AD, tau undergoes changes that cause it to gather together abnormally in tangled filaments in neurons (for more on this, see the section A Walking Tour Through the Brain). In studying tau and what can go wrong, investigators have found that tau abnormalities are also central to other rare neurodegenerative diseases. These diseases, called tauopathies, include frontotemporal dementia, Pick's disease, supranuclear palsy, and corticobasal degeneration. They share a number of characteristics, but also each have distinct features that set them apart from each other and from AD. Characteristic signs and symptoms include changes in personality, social behavior, and language ability; difficulties in thinking and making decisions; poor coordination and balance; psychiatric symptoms; and dementia. Recent advances include the discovery of mutations in the tau gene that cause one tauopathy called frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17). The development of several mouse models that produce tau tangles, will allow researchers to address the many questions that remain about these diseases. The development of a "double transgenic" mouse that has both tau tangles and beta-amyloid plaques will also lead to further insights about AD.

Cardiovascular Risk Factors Several recent studies in populations have found a possible link between factors related to cardiovascular disease and AD. One of these studies found that elevated levels of an amino acid called homocysteine, a risk factor for heart disease, are associated with an increased risk of developing AD. The relationship between AD and homocysteine is particularly interesting because blood levels of homocysteine can be reduced by increasing intake of folic acid and vitamins B6 and B12. In fact, in other studies, scientists have shown that folic acid may protect against nerve cell loss in brain regions affected by AD. Investigators have also found that the use of statins, the most common type of cholesterol-lowering drugs, is associated with a lower risk of developing AD.

Oxidative Damage from Free Radicals Another promising area of investigation relates to a longstanding theory of aging. This theory suggests that over time, damage from a kind of molecule called a free radical can build up in neurons, causing a loss in function. Free radicals can help cells in certain ways, such as fighting infection. However, too many can injure cells because they are very active and can readily change other nearby molecules, such as those in the neuron's cell membrane or in DNA. The resulting molecules can set off a chain reaction, releasing even more free radicals that can further damage neurons. This kind of damage is called oxidative damage. It may contribute to AD by upsetting the delicate machinery that controls the flow of substances in and out of the cell. The brain's unique characteristics, including its high rate of metabolism and its long-lived cells, may make it especially vulnerable to oxidative damage over the lifespan. Some epidemiological and laboratory studies suggest that anti-oxidants from dietary supplements or food may provide some protection against developing AD. Other studies suggest that low calorie diets may protect against the development of AD by slowing down metabolic rates.

Inflammation Another set of hints about the causes of AD points to inflammation in the brain. This process is part of the immune system and helps the body react to injury or disease. Fever, swelling, pain, or redness in other parts of the body are often signs of inflammation. Because cells and compounds that are known to be involved in inflammation are found in AD plaques, some researchers think it may play a role in AD.

They disagree, though, on whether inflammation is a good or a bad thing. Some think it is harmful - that it sets off a vicious cycle of events that ultimately causes neurons to die. Evidence from many studies supports this idea.

Other scientists believe that some aspects of the inflammatory process may be helpful - that they are part of a healing process in the brain. For example, certain inflammatory processes may play a role in combating the accumulation of plaques. Many studies are now underway to examine the different parts of the inflammatory process more fully and their effects on AD.

Brain Infarction We've all heard the sensible advice about ways to live a long and healthy life: eat right, exercise, don't smoke, wear a seat belt. All of these habits can help prevent heart attacks, stroke, and injuries. This advice may even have some relevance for AD as well. Results from one long-term study of aging and AD show that participants who had evidence of stroke in certain brain regions had more symptoms of dementia than could be explained by the number of plaques and tangles in their brain tissue. These findings suggest that damage to blood vessels in the brain may not be enough to cause AD, but that it could make AD clinical symptoms worse.


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B. New Techniques Help in Diagnosing AD

A healthy man in his early 60s begins to notice that his memory isn't as good as it used to be. More and more often, a word will be on the tip of his tongue but he just can't remember it. He forgets appointments, makes mistakes when paying his bills, and finds that he's often confused or anxious about the normal hustle and bustle of life around him. One evening, he suddenly finds himself walking in a neighborhood a couple of miles from his house. He has no idea how he got there.

Not so long ago, this man's condition would have been swept into a broad catch-all category called "senile dementia" or "senility." Today, the picture is very different. We now know that Alzheimer's and other illnesses with dementia are distinct diseases. Armed with this knowledge, we have rapidly improved our ability to accurately diagnose AD. We are still some distance from the ultimate goal - a reliable, valid, inexpensive, and early diagnostic marker - but experienced physicians now can diagnose AD with up to 90 percent accuracy.

Early diagnosis has several advantages. For example, many conditions cause symptoms that mimic those of Alzheimer's disease. Finding out early that the problem isn't AD but is something else can spur people into getting treatment for the real condition. For the small percentage of dementias that are treatable or even reversible, early diagnosis increases the chances of successful treatment.

Even when the cause of the dementia turns out to be Alzheimer's disease, it's good to find out sooner rather than later. One benefit is medical. The drugs now available to treat AD can help some

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